The present invention relates to a spectrally sensitized silver halide photographic material, more particularly to a silver halide photographic material spectrally sensitized to the near infrared region suitable for use with a lasing light source.
In medical diagnosis, an attempt is being made at performing computerized image processing on an input digital radiation image to provide data which is adapted for diagnosis, and then producing a reconstructed image by exposure with a laser beam. Various lasers including argon, helium-neon and helium-cadmium lasers have been commercialized for use as light sources in scanning exposure systems but they have several defects such as short life, bulkiness and the need to employ a complicated device for modulating radiation intensity. Compared with these lasers, semiconductor lasers are inexpensive, long lived, small in size and have the advantage of obviating the need to employ a modulator because of their capability for direct tuning. In addition, a further increase in the power and life of semiconductor lasers has been successfully realized by recent studies.
Semiconductor lasers having the features described above emit light in the near infrared range of the spectrum (750-1500 nm), so recording materials that are to be used with semiconductor lasers must have sensitivity in the near infrared range. While various methods have been known for sensitizing the silver halide in a silver halide photographic material to the near infrared portion of the spectrum, one approach is to use long-chain cyanine dyes (see The Theory of the Photographic Process, 3rd Ed., Macmillan, 1966, pp. 198-201). However, light-sensitive materials spectrally sensitized with such cyanine dyes are poor in keeping quality and will suffer from substantial desensitization unless stored at low temperatures. In order to prevent desensitization during storage, three methods may be adopted: one is to increase the concentrations of silver and hydrogen ions in the photographic emulsions used in light-sensitive materials; another way is to reduce the use of restrainers; and the third method is the combination of these two. Either method is effective in preventing desensitization but on the other hand, increased fog will occur. To avoid this problem, the following obvious methods may be adopted: the concentrations of silver and hydrogen ions are reduced; the use of restrainers is increased; or these methods is combined. However, all of these approaches suffer from the disadvantage of reduced sensitivity. It has therefore been considered difficult to prevent both increased fog and reduced sensitivity simultaneously during storage.
Light-sensitive materials spectrally sensitized to the near infrared region have the added disadvantage that when subjected to high-speed processing, for example, processing that is completed within the period of 20-60 seconds, increased fog and graininess will occur. The problem could be solved by using restrainers in a greater amount but then, as already mentioned, desensitization will occur during storage.